Vehicular battery chargers which run off vehicular batteries and supply electrical current to charge electronic devices such as battery operated radios are well known. Vehicular chargers are normally installed inside the passenger compartment of a vehicle (e.g. police car) in order to allow the radio user the capability of recharging the radio battery while inside the vehicle.
Present day battery chargers allow the radio user full use of the radio while the radio is being charged. Some of the more sophisticated radio chargers such as the SABER.TM. Vehicular Adapter manufactured by Motorola, Inc. also includes features such as a built-in audio amplifier to increase the volume of any incoming radio calls while the radio is in the charger, as well as external annunciators and a remote microphone which allow the radio user full use of the radio while the radio is still charging.
The major drawback of present day vehicular chargers can be best illustrated by looking at FIG. 1 which shows the typical operating cycle of most present day chargers. A typical vehicular charger will first determine if the vehicle's ignition switch is in the "on" position (either in the accessory position or with the car running), as shown in decision step 102. If the ignition switch is in the "off" position, the battery charging will be discontinued as shown in step 104, and any ongoing radio charging will be immediately terminated. On the other hand, if the charger determines that the ignition switch is in the "on" position, the charger will enable the charging of any radio which is inserted into the charger as is shown in step 106. The major problem with the approach in FIG. 1 is that if a radio user leaves his radio in the charger while the radio is "on" and at the same turns "off" the ignition switch, the radio battery will begin to be discharged once the ignition switch is turned off and the radio user may come back to find a radio with a discharged vehicle battery.
Other types of present day chargers bypass the ignition switch of the vehicle altogether and connect directly to the vehicle's battery. The problem with these types of chargers is that if a radio is left inside the charger for a prolonged period of time without the vehicle being turned "on" there is a possibility of the vehicle's battery becoming discharged due to the current drain caused by the charging of radio.
A need exists in the art for a vehicular charger which can overcome the previously mentioned problems of either discharging the vehicle or radio batteries under certain situations. The charger must allow for a radio to be automatically charged while the vehicle is turned "off", yet at the same time not allow for the vehicle's battery to be depleted to the point that the vehicle will not be able to be started if the radio is left in the charger.